This invention relates to a digital signal regenerator which may be more particularly described as a regenerator for regenerating a quaternary, or four-level, signal.
In the prior art, signal regenerators used in digital transmission systems typically regenerate a bipolar return-to-zero signal. Such a bipolar signal represents transmitted logic ones as alternate positive and negative polarity pulses having a level of one. Between transmitted pulses, the signal returns to the zero level. Transmitted logic zeros are represented by the zero level during a pulse sample time.
The transmission rate for such a bipolar return-to-zero signal is limited to one bit of information during each time slot. This maximum transmission rate is determined on exchange grade cable pairs by cross-talk coupling paths between conductors. Coupling path loss decreases with increasing frequency. As a result, interference between signals at higher transmission rates degrades system performance.
Some digital transmission routes have experienced enough growth in communications needs that the cable facility either is filled to capacity or is nearing full capacity. In either case there is a problem in providing route relief.
One way to provide relief is to place a new cable along the route. In view of the high cost of providing copper wire pairs, this is a very expensive solution.
Another way to provide relief is to replace the electronics of the digital transmission system with electronics which will transmit the same amount of information more quickly. If a higher pulse rate is chosen for increasing the amount of information transmitted during a predetermined interval, it is necessary to operate at a higher frequency and to reduce repeater spacings along the cable route. Cable loss between repeaters, level differences between transmitted and received signals, and cross-talk are all reduced because of reduced repeater spacings. Such reduced repeater spacings, however, increase the number of repeaters and the cost per channel mile.
What is needed to provide economical relief is a digital transmission system that increases the rate of information flow without substantially increasing the frequency of operation. Such a system should use the existing cable, retain existing repeater spacing, and reuse existing repeater sites while providing increased capacity. There is a problem in designing a line regenerator which will transmit more information during a predetermined time without increasing the line frequency.